Add ability to set group ID on generated QRcode

It would be nice if we could manually define the ID of the generated QRCode. This will help me to batch process some files generation and will allow me to select the newly added object in inkscape --actions="select:the_id …".

I think a new argument could be added to the script, let's call it groupid. If empty or in case of conflit, the ID must be auto-generated as before.

MR is coming.